The Jurassic sequences in the South Qiangtang Basin of Tibet are essential for understanding the paleogeography and tectonic evolution of this basin and the Bangong-Nujiang Meso-Tethys Ocean (BNMO). However, the partial absence of fossils hinders the study of the stratigraphic distribution and tectonic paleogeography of the basin. Late Triassic bivalves, including nine species in nine genera, were identified for the first time in olistostromes from the Sêwa Formation in the Riganpeicuo area. Based on detailed geological field surveys and sedimentary facies analysis, the lower-middle part of the Rigenco section is a deep-water turbidite fan deposit and the upper part is a shallow sea mixed shelf deposit, which differs distinctively from the carbonate platform facies of the Jiebuqu Formation. Therefore, the strata formerly assigned to the Jiebuqu Formation (Rigenco section) is formally reassigned to the Lower-Middle Jurassic Sêwa Formation and Middle Jurassic Shaqiaomu Formation. These results not only provide further understanding of the sedimentary infill history of the South Qiangtang Basin, but also further support that the BNMO was already open in the Late Triassic and formed a mature ocean in the Early-Middle Jurassic, with subsequent rapid subduction. Both the opening and subduction processes were completed in a relatively short time interval with a large number of attendant olistostromes.

The Upper Carboniferous Zhanjin Formation has attracted much attention from geoscientists for containing glacial–marine diamictite and cold‐water fauna typified by the bivalve Eurydesma. The presence of this Formation has provided important evidence for determining the northern border of Gondwana. Previous researchers have classified those strata north of Niangrong Co in the Gêrzê region as part of the Zhanjin Formation based on the presence of glacial–marine diamictite, although the absence of biological fossil evidence has defied clear age determination. Our field investigations first discovered large quantities of corals, sponges and bryozoans. All coral fossils were identified as belonging to the Hexacorallia subclass including 13 genera and 25 species, primarily including Conophyllia guiyangensis Deng et Kong, Coryphyllia regularis Cuif, Cuifia columnaris Roniewicz, Distichophyllia norica Freeh, Distichophyllia gigas Vinassa de Regny, Pamiroseris rectilamellosa Winkler, Retiophyllia clathrata Emmrich, and Retiophyllia paraclathrata Roniewicz. Extensive biostratigraphic correlations show that the hexacorallia should belong to the Late Triassic, thereby negating the presence of the Zhanjin Formation in the study area. Based on analyses of sedimentary facies and detailed study of the glacial–marine diamictite as supposed by earlier researchers, the limestone blocks and gravels within the facies are slope facies olistostromes and waterway sediments from lime slurry debris flows in the submarine fan rather than primary sedimentary products. Among them, lenticular sandstone should be sequentially distributed waterway sand bodies, indicating that the strata have no glacial–marine diamictite. In addition, the rocks containing the mentioned fossils are just limestone blocks from olistostromes, and limestone gravels from waterways of submarine fans. Such a result further negates the presence of the Zhanjin Formation in the study area, and indicates that the age of the studied strata should be youner than the Late Triassic. Through regional stratigraphic comparisons and the study of tectonic settings of the strata, the sedimentary characteristics of the subject strata, including lithology, lithofacies and fossils, are confirmed to be similar to the widely distributed Sêwa Formation in this region. We thus infer that the strata belong to the Middle–Lower Jurassic Sêwa Formation. This finding is important for both studying paleogeography of Tibet and determining the northern boundary of Gondwana.

The evolution patterns of the Neo-Tethys Himalayas have been a major topic of research, particularly in the Neo-Tethys Ocean. The geological field investigations were conducted in the Late Jurassic-Early Cretaceous Sangxiu Formation in the Tsomei Longzi area of Tibet. A stratigraphic hierarchy of the Sangxiu Formation was established based on an analysis of the sedimentary lithology in this area. Based on the geochemical characteristics and chronology of the felsic-mafic volcanic rocks of the Sangxiu Formation, the genesis, tectonic background, and evolutionary pattern of the volcanic rocks of the Sangxiu Formation were revealed. Basalts, dolerite, and volcanic debris constitute the volcanic rocks of the Sangxiu Group in the Zhegu area. The Early Cretaceous Sangxiu Formation basalts were determined using SHRIMP zircon U-Pb ages of 141 ± 1 Ma and 142 ± 1 Ma. Volcanic rocks of the Sangxiu Formation, which are intraplate rifting products of the Late Jurassic-Early Cretaceous period, originated from the mantle and mixed with crustal materials. The rock type is an intraplate alkaline basalt that formed during the rifting activity of the passive continental margin extension. There was a crucial growth episode in the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous. The Neo-Tethys Ocean expansion from the Late Triassic to the Early Cretaceous was caused by a younger rifting along the passive continental edge rather than a continuation of the early Mid-Ocean Ridge development, thus demonstrating the expansion of the Neo-Tethys Ocean at various stages.